1. Field of the Invention
The present invention relates to an ultrathin-walled rolling bearing used in industrial robots, machine tools, medical instruments, and the like, and more particularly it relates to an arrangement for stabilizing the behavior of a segment type cage of resin incorporated into an ultrathin-walled rolling bearing, so as to reduce the rubbing sound that is produced when the cage comes in contact with the bearing rings.
2. Description of the Prior Art
In FIG. 8, an example of a CT scanner device that is a kind of medical instrument is shown. In the CT scanner device, X-rays produced by an X-ray tube device 50 are applied to a subject 53 through a wedge filter 51 for uniformizing their intensity distribution and a slit 52 for restricting the intensity distribution. The X-rays passing through the subject 53 are received by a detector 54, where they are converted into an electric signal, which is then fed to an unillustrated computer. Such parts as the X-ray tube device 50, wedge filter 51, slit 52, and detector 54 are mounted on a substantially cylindrical rotary frame 57 rotatably supported on a fixed frame 56 through a bearing 55, the rotary driving of the rotary frame 57 rotating the parts around the subject 53. In the CT scanner device, the rotary motion of the mutually opposed X-ray tube device 50 and detector 54 around the subject 53 provides the projection data covering all angles in the examination cross-section of the subject 53, and a tomographic image is obtained from a reconstructed program programmed in advance from these data.
In the CT scanner device, since the inner periphery of the fixed frame 56 is formed in a size (about 1 m in general) to receive the subject 53, a bearing that is very small in cross-sectional area for its diameter, or a so-called ultrathin-walled rolling bearing is used as the bearing 55 between the fixed frame 56 and rotary frame 57.
Those ultrathin-walled rolling bearings which are frequently used in CT scanner devices have their cages made of resin. Such resin-made cage is in the form of a plurality of arcuate segments annularly joined together, wherein adjacent segments projection-recess fit together at their ends. Each segment is an injection-molded article, and fiber-reinforced polyamide resin (PA66) is generally used as a raw material therefor. The PA66 has a greater linear expansion coefficient than that of bearing steel, which is a material for raceway rings, and the PA66 has a property such that its dimensional difference increases according to temperature change or such that it expands due to its water absorption. Therefore, in the case of a large-sized bearing, the circumferential length of the cage greatly changes. With all segments joined together, the change in the circumferential length of the cage can eliminate the guide gap between the cage and the raceway rings; therefore, the cage is used with a gap G opened at one place (the non-joined state) (FIG. 10), rather than all the segments being joined together.
The prior art of this kind of ultrathin-walled rolling bearing is described, for example, in Japanese Patent applications Laid-Open under Nos. 2000-329143, 2001-304266, and 2002-81442.
Since resin-made segments constituting a cage are injection-molded articles, there are cases where the fitting in the convex-concave fit portion becomes loose due to an error in molding. Loose fitting can result in a dislocation occurring between adjacent segments 410 and 420, as shown in FIG. 9, which illustrates a case where the right-hand side segment 420 is dislocated radially inward.
The cage 400 of the ultrathin-walled rolling bearing is used generally with a raceway ring guide; in FIG. 9, a case of an outer ring guide is illustrated. The raceway ring-guided cage is radially guided (centered)by the cage guide surface of the raceway ring (either the outer ring inner diameter surface or the inner ring outer diameter surface) and is adapted to properly maintain the radial clearance between the cage guide surface of the raceway ring and the cage outer diameter surface (or inner diameter surface) to prevent the whirling of the cage. And, with the rotation of the bearing, the cage rotates by receiving driving power from the outer ring 100, inner ring 200 or rolling elements 300 depending on the guide system while radially moving within the range of the guide clearance S in the cage indicated by S in FIG. 9. Therefore, in the case of the outer ring-guided cage, for example, when a dislocation occurs in the segment 420, as shown in FIG. 9, with the rotation of the cage 400 the end 421 of the segment 420 contacts the outer periphery of the inner ring 200, which it should ordinarily do not contact, (hereinafter referred to as “abnormal contact”), which sometimes forms a factor of abnormal sounds, such as tapping sound and vibroacoustics.
Further, the lead and delay of the rolling elements take place depending on the internal clearance of the bearing and the service conditions. If, as in the prior art, the cage is used with a gap G opened at one place in the cage (the non-joined state) (FIG. 10), rather than all the segments being joined together, the lead and delay of the rolling elements make the behavior of the cage unstabilized, thus easily leading to the rubbing sound that occurs when the cage contacts the raceway, or the tapping sound that occurs when the segment in the non-joined region taps the raceway rings. Such rubbing sound becomes a problem in the bearings used in CT scanners or the like required to rotate at high speed and quietly.